Many different kinds of vaccines are being employed to prevent pathogenic entities to enter the body or to prevent the pathogenic entities to spread and cause illnesses. Vaccines that are being applied nowadays and/or vaccines that are being tested in different stages of development include whole-inactivated viruses, (live-) attenuated viruses, peptide vaccines, (naked) DNA vaccines, sub-unit vaccines and vaccines that are based on (relatively) harmless viruses that harbor an antigenic determinant from the pathogenic entity towards which the vaccine is directed. Examples of such “vaccine carriers” are influenza virus, alphaviruses such as Semliki Forest Virus or Sindbis virus, and adenoviruses. Wild-type adenoviruses are known to cause relatively mild diseases such as common colds. To date, over 50 different adenovirus serotypes have been identified, subdivided into six subgroups based on their sequence homologies and hemagglutination abilities. Recombinant adenoviruses are being extensively tested in HIV vaccine clinical trials and in vaccines against malaria (WO 01/02607; WO 02/22080; WO 01/21201; Sullivan et al. 2000; Shiver et al. 2002). The results that were obtained in these studies clearly show that adenoviruses provide an excellent tool for delivery of the antigen to the host. One could envision an endless list of other pathogens that could be targeted by using the adenovirus as an antigen carrier providing proper protection. Such pathogens include, but are not limited to, viruses, bacteria, yeasts, fungi, etc.
However, a few important drawbacks exist when the most common and probably the best-studied adenovirus serotype, Adenovirus 5 (Ad5) is used. As has been described extensively elsewhere (PCT International Publication WO 00/70071), it is known that most people across the world have encountered an Ad5 infection at least once in their life. This results in a level of neutralizing antibodies that is relatively high and causes a rapid clearance from the system. Moreover, it is known that almost all Ad5-derived recombinant vectors end up in the liver. This phenomenon presumably prevents the recombinant vector (based on Ad5) from very efficiently entering the antigen-presenting cells such as dendritic cells. The art has recognized that there was a need for alternative adenoviruses that would not home to the liver, but rather would be targeted to the cells involved in the immune system. One way of triggering this was by employing the receptor- or cell-binding moiety of the adenovirus. This moiety was swapped from certain adenoviruses not having a tropism for liver cells to Ad5. An example of such a recombinant adenovirus is Ad5fib16, which is a recombinant adenovirus based on Ad5, but carrying the tropism-determining part of the fiber of adenovirus serotype 16 in its capsid (see PCT International Publications WO 00/03029 and WO 02/24730).
Nevertheless, significant problems remain to be solved. Many of these are based on the finding that an infection (and injection) with a specific adenovirus elicits a significant immune response in humans and thereby hampers different kinds of vaccinations, using that same specific adenovirus serotype. Thus, if an individual has encountered a specific serotype, it is in general hard to obtain an immune response by using a vaccine based on that particular serotype. This would, therefore, limit the possible use of recombinant adenovirus as an antigen carrier for vaccination purposes.